1. Field of the Invention
The present invention relates to a magnetic circuit and a speaker and particularly relates to a magnetic circuit which includes only a magnet without a yoke, which is thin and light in weight and which has a symmetrical structure with respect to the vibration direction, and to the improvement of a speaker using this magnetic circuit.
2. Description of the Related Art
Conventionally, as a thin type speaker or magnetic circuit for a speaker, it is known one disclosed in patent literature 1 in which thin disk-shaped or ring-shaped two sheets of magnets with the same polarity of S, S (N, N) are arranged opposing each other with a spacer made of a non-magnetic material positioned in between.
FIG. 1 shows a vertically sectional view of a speaker disclosed in Japan Published Utility Model Application No. H2-30957, in which the magnetic circuit includes: first and second ring-shaped magnets 1 and 1 magnetized in the direction of upper and lower surfaces and disposed whose magnetized surfaces of the same polarity such as S, S (N, N) oppose each other with a spacer 6 made of a non-magnetic disk-shaped material in between, and a bobbin hangs down from a diaphragm 5 to the outer circumference of the spacer 6, around which a voice coil 3 is wound. Further, at approximately the center position of the spacer 6 a third and a fourth disk-shaped magnets 1a and 1a formed of the same small magnetic circuit as described above through a cylindrical shaped spacer are magnetized in the direction of the upper and lower surfaces and are disposed whose magnetized surfaces of the same polarity such as S, S (N, N) oppose each other with a spacer 6a made of a non-magnetic disk-shaped material in between, and a bobbin hangs down from a diaphragm 5a to the outer circumference of the spacer 6a, around which a voice coil 3a is wound to form a coaxial type speaker. A vibration system including the diaphragms 5, 5a and the edges 4, 4a is held by a frame 7.
Further, a speaker is disclosed in Japan Published Patent Application No. H6-233384, in which a repulsion magnetic field is formed by arranging two sheets of magnets in the magnetic directions which repel each other; the voice coil wound approximately around the center of the outer circumference of a voice coil bobbin is disposed in this repulsion magnetic field; the inner circumference portion of a diaphragm is connected to the center of the outer circumference of this voice coil; also both the end portions of the outer circumference of the above voice coil bobbin are supported by two sheets of dumpers which are disposed at an interval to each other; and a support system of this voice coil is formed in a symmetrical-shape with a center axis perpendicular to the direction of an axis of the voice coil to be an axis of symmetry.
FIG. 2 is a schematic diagram showing a winding method of the voice coil in the vicinity of a magnetic circuit in the speaker disclosed in Japan Published Patent Application No. H6-233384, and the repulsion magnetic field is formed by arranging two sheets of magnets 1b, 1b in the magnetic directions which repel each other with a spacer 6 in between. In such a magnetic circuit, a strong magnetic field is generated in a portion of the leakage magnetic flux Φ1 between the magnets 1b and 1b, and a magnetic field in the reverse direction is generated in the portions of each leakage magnetic flux of Φ2 and Φ3 around the magnets 1b and 1b. 
Then, as shown in FIG. 2, the voice coil 3 is formed by arranging a first coil 3a wound at the position approximately corresponding to one magnet 1b, a second coil 3b wound at the position approximately corresponding to the spacer 6 and a third coil 3c wound at the position approximately corresponding to the other magnet 1b in the outer circumference of a voice coil bobbin 8 in a row. Further, as shown with an arrow in FIG. 2, the winding direction of the first and third coils 3a and 3c is a reverse direction to the winding direction of the second coil 3b. 
By arranging the voice coil 3 in the repulsion magnetic field formed by the magnets 1b and 1b and the spacer 6, a driving force can be obtained from the second coil 3b in the leakage magnetic flux Φ1 portion generating a strong magnetic field, and a driving force can be obtained by the first and third coils 3a and 3c from each leakage magnetic flux portion of Φ2 and Φ3 generating a magnetic field of a reverse direction. Therefore, there are disclosed that with the portion constituting the magnetic circuit being thinner and lighter in weight, a large driving force can be obtained, and a constant impedance characteristic can be obtained by the inductance reduction effect mutually generated in the coils, so that a high frequency region can be improved efficiently.
However, each of the above described conventional art has a structure in which a magnetic circuit including the magnets 1, 1a and 1b, and the spacer 6 made of a non-magnetic or ferromagnetic material are arranged on the inner side of the voice coil bobbin 8. Therefore, a structure to support a magnetic circuit is necessary, and a speaker which has a symmetrical vibration system with respect to a center axis that is perpendicular to the direction of a center axis of the voice coil 3 can not be obtained. As a result, there is no symmetry in the vibration direction of the diaphragms 5 and 5a, which causes the distortion of acoustic signals.
In order to obtain a vibration system that is symmetrical with respect to the vibration direction, it is considered to form a magnetic circuit of a inside magnetic type in which the voice coil is arranged inside the frame-shaped or ring-shaped magnetic circuit formed of the frame-shaped or ring-shaped two sheets of magnets 1, 1a, 1b making the same polarity opposing each other with a spacer made of a frame-shaped or ring-shaped non-magnetic or ferromagnetic material in between.
In a speaker of such magnetic circuit construction, it is possible to obtain a speaker having a symmetrical structure in the vibration direction by supporting the outer circumference portion of a frame-shaped or ring-shaped magnetic circuit from the outside.
However, in a frame-shaped or ring-shaped magnetic circuit, magnetic flux converges in the outer circumference of a frame-shaped or ring-shaped magnetic circuit, and this magnetic flux becomes useless because it does not contribute at all to drive a speaker.
Further, in such magnetic circuit, when using a spacer 6 made of non-magnetic material, the magnetic energy (effective magnetic flux) which the magnets 1, 1a and 1b have can not be taken out efficiently.
Furthermore, though the magnetic energy can be taken out efficiently when using a spacer 6 made of a ferromagnetic material, the magnetic flux density of the spacer 6 made of a ferromagnetic material is saturated when a magnet having a large magnetic energy is used, resulting in difficulty in which an effective magnetic flux cannot be taken out efficiently.
The present invention has been made in order to solve the above described problems, and the purpose of the present invention is to obtain a speaker which is thin and light in weight and which has a symmetrical vibration system with respect to a center axis which is perpendicular to the direction of the center axis of the voice coil, and to provide accordingly a speaker which can obtain the high quality sound.
Another purpose of the present invention is to improve the rate of use for a magnet by efficiently using a strong leakage flux in a thin and light-weight magnetic circuit whose magnetic path is not closed, and to provide a magnetic circuit without a saturation phenomenon of the magnetic flux caused by components (spacer, plate, yoke and so on) which constitute a magnetic circuit.
Further another purpose of the present invention is to provide a magnetic circuit and speaker in which a plurality of coils are combined to reduce the inductance component of the input impedance of the voice coil, so that the amplitude of vibration of a constant input impedance speaker and a diaphragm is expanded.